I am building a Web API that calculates a loan. This loan has a list of payments:

@Entity
public class Loan {
    
    @Id
    @GeneratedValue
    @Column(name = "id", nullable = false)
    private Long id;

    ...

    @OneToMany(cascade = CascadeType.ALL)
    private List<Payment> amortizationSchedule;

}


@Entity
public class Payment {
    @Id
    @GeneratedValue
    private Long id;

   ...

}

Whenever I store these entities I would expect the primary keys to be generated as follows:

1. Loan created, generated id = 1
2. Loan created, generated id = 2
3. Payment created, generated id = 1
4. Payment created, generated id = 2

Instead the following happens:

1. Loan created, generated id = 1
2. Loan created, generated id = 2
3. Payment created, generated id = 3
4. Payment created, generated id = 4

Why is this happening? And how can I get the Java Persistence API to base the primary key generation on the amount of entities of a certain class instead of the total amount of entities?

CodePudding user response：

Try adding a strategy to your generationvalue

@GeneratedValue(strategy = GenerationType.IDENTITY) 

CodePudding user response：

The default @GeneratedValue in Hibernate is AUTO, but since hibernate 5 the default interpretation of AUTO has changed and is using a sequence generator. This is why you were seeing consecutive number for different tables, because the ids came from the same source, a sequence (and if you are using MySQL, and I suspect you do, which doesn't support SEQUENCE, you would notice that hibernate had created a table hibernate_sequence to emulate it).

https://docs.jboss.org/hibernate/orm/5.0/userguide/html_single/Hibernate_User_Guide.html#identifiers-generators-auto

This is the default strategy since Hibernate 5.0. [...] . When using this strategy, AUTO always resolves to SequenceStyleGenerator. If the underlying database supports sequences, then a SEQUENCE generator is used. Otherwise, a TABLE generator is going to be used instead.

To fix this just do what @CodeAddict said in his answer, use @GeneratedValue(strategy = GenerationType.IDENTITY) so it uses the AUTO_INCREMENT feature of MySQL.